1. Field of the Invention
The present invention relates to the use of radiation to measure the distance and velocity of an object of interest. More particularly, the present invention relates to a charge integration ranger which utilizes radiation pulses, knowledge of the velocity of light and a charge transfer device detector to measure the distance and velocity of an object and provide three-dimensional imaging.
2. Description of the Related Art
Telemetry systems which determine object distance using optical radiation are known. Such systems send radiation towards a distant object and measure distance based on the reflected radiation that returns to the system. Distance is determined based on one of several types of calculations, including triangulation, frequency changes in the returned radiation, and the time the optical radiation takes to reach the object and return to the system.
One technique for measuring distance and velocity that has received widespread attention in recent years is heterodyne or coherent detection. In heterodyne detection systems, the frequency of an emitted optical radiation signal is modulated so that optical heterodyne detection can be employed By its very nature, optical heterodyne detection implies that some sort of interferometer is employed in the receiver for aiding in phase comparison The main advantages of heterodyne detection include narrowband signals, a high signal-to-noise ratio (SNR), shot-noise-limited performance and the ability to measure Doppler frequency shift in order to determine radial velocity Additionally, optical heterodyne detection requires a relatively low energy optical radiation source, permits detection at the quantum noise limit, and is theoretically more sensitive than known direct detection techniques.
In practice, heterodyne detection poses a number of difficulties that may, even under mildly adverse conditions, severely affect performance For example, large object velocities require high detector bandwidths. High bandwidth detectors normally have a higher noise equivalent power and other undesirable characteristics. Additionally, without some prior knowledge of the signal location, narrow bandwidth signals at high frequencies require long search times or a large number of tuned filters Further, interferometers are extremely sensitive devices which require precise phase front control. Any sort of unwanted phase perturbation, whether produced by or in the interferometer or by the laser source feeding the interferometer, will degrade the signal and can be considered noise. Many other sources of noise also exist While some of the noises, such as detector, background, electronic and local oscillator shot noises, can be effectively overcome by using a large local oscillator, a number of other noise sources persist. These sources include modulation instability and distortion, optical misalignment and imperfections, vibrations, atmospheric turbulence, object vibration and rotation, and reflection modulation.
So while heterodyne detection theoretically offers a great number of advantages, it is not presently practical for many applications. Extensive engineering is required to minimize the noise sources, and these required noise reduction techniques are complex, difficult to implement and expensive. Further, such techniques may require continuous monitoring, which further reduces the applicability of heterodyne detection.
So-called "direct" detection is widely used in range finding systems. Simple implementations of direct detection systems employ an array of light-sensitive diodes which has a known mathematical relationship with a radiation source so that the distance of an object can be determined by trigonometric methods, such as triangulation. Quite commonly, these systems are used in cameras for determining the range of an object to be photographed for focusing an automatic lens. However, such systems have limited applications because of limited range capability and relatively poor range resolution.
For example, U.S. Pat. No. 4,522,492 to Masunaga discloses a distance measuring device which includes an infrared emitting portion having an infrared emitting diode and a projection lens, and an infrared receiving portion which includes a receiving lens and a line sensor comprising light-sensitive elements. Since this system measures distance by triangulation, the object to be measured must be positioned along the optical axis of the infrared emitting portion. Additionally, a line from the emitting lens to the receiving lens is perpendicular to the optical axis, and the two lenses must be a known distance apart. Distance to the object is calculated based upon which light-sensitive element in the line sensor receives the infrared light reflected from the object, as the two non-right angles in a right triangle created with the object can be determined, since the distance between the light-sensitive element and the optical axis is known. However, applications of such a system are limited to situations where the exact direction of an object to be ranged is known, since the object must be aligned along the optical axis of the infrared emitting portion. Further, only distance can be measured.
A second direct detection system is disclosed in U.S. Pat. No. 4,746,790 to Sorimachi. In the Sorimachi system, a pair of light receiving portions are utilized each comprising a lens and an array of light-sensitive elements The arrays can be comprised of charge-coupled device (CCD) arrays. The object to be ranged is aligned along the optical axis of a first portion, and the lens of the second portion, the CCD array, or both are moved until light reflected from the object is received by a predetermined element of the CCD array of the second portion The distance between the lenses is known, and any movement of the lens and/or the array is measured so that the distance of the object can be determined by triangulation. This system has the same limitations as the Masunaga system.
Other direct detection systems measure the phase shift of the return from an amplitude modulated source (tone ranging) or the round trip time of a very narrow pulse. However, all of the known direct detection systems suffer from a number of drawbacks. For example, known direct detection systems suffer from relatively low SNR because of the high electronics noise associated with processing the signal. Accordingly, sensitivity of direct detection systems has been relatively poor. Further, large electrical bandwidths are required for the known pulse direct detection systems. Pulse integration could reduce the system bandwidth, but pulse integration results in the integration of both signal and noise, resulting in a less than optimum SNR or sensitivity. Low-sensitivity translates into a need for relatively high optical powers. High optical powers pose an eye safety problem, and such systems require a high energy radiation source. Among other problems, higher power generation requirements tend to reduce portability. All of these factors reduce the number of possible applications of direct detection systems.
A great deal of research has been performed in the field of heterodyne detection because of the deficiencies in the existing direct detection techniques. However, these efforts have yet to produce a heterodyne system capable of widespread application. Accordingly, a need exists for a range detection system which has the low-noise characteristics, simplicity and reliability of direct detection systems and the high sensitivity, high SNR and narrow bandwidths theoretically available in heterodyne detection systems.